1. Field of the Invention
The present invention relates to a honeycomb structure suitably used particularly as a carrier for catalyst for purification of automobile exhaust gas, as well as to a method for fixing the honeycomb structure. More particularly, the present invention relates to a hexagonal-cell honeycomb structure of large mechanical strength, good purifiability for exhaust gas and small pressure loss, as well as to a method for fixing the honeycomb structure.
2. Description of Related Art
In recent years, regulation for exhaust gas has been intensified in various countries for reasons of increasing movement for protection of global environment, etc. In this connection, improvements of engine itself have been made to reduce the amount of harmful substances [e.g. hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx)] discharged from engines; also, improvements of three-way catalyst (three-way catalysts are currently a main stream of the catalysts for exhaust gas purification) are being continued. Owing to these two improvements, the amount of harmful substances discharged is being steadily decreased.
Thus, the overall amount of harmful substances discharged in ordinary running operation is being reduced. Meanwhile, attention is being paid to the amount of harmful substances discharged right after the start of engine. For example, 60 to 80% of the total harmful substance amount discharged in total running cycles is discharged within 140 seconds from the start of engine when measured according to FTP-75 (regulated running cycles of U.S.), and this level is regarded as a problem.
One reason of the result according to FTP-75 is that right after the start of engine, since the temperature of exhaust gas is low and the catalyst used is not activated sufficiently, the harmful substances in exhaust gas pass through the catalyst without being purified. Another reason is that right after the start of engine, the combustion in engine is not stabilized and the A/F ratio (air-to-fuel ratio) in exhaust gas (which is an important parameter for determining the purifiability of three-way catalyst), i.e. the proportion of oxygen in exhaust gas is under fluctuation.
Catalysts exhibit the highest purifiability when the A/F ratio is a theoretical (stoichiometric) air-to-fuel ratio of 14.7. Therefore, since it is preferable to quickly increase the catalyst temperature of right after the start of engine, it has been attempted to (1) provide a catalyst at a position closer to engine where the temperature of exhaust gas is higher, (2) reduce the heat capacity of honeycomb structure (which is a catalyst carrier), or (3) increase the cell density of honeycomb structure in order to (a) allow the honeycomb structure to absorb the heat of exhaust gas quickly and (b) increase the contact area between catalyst and exhaust gas.
Also, in engines, improvements have been made to allow the A/F ratio to reach a theoretical air-to-fuel ratio as quickly as possible. Meanwhile, in catalysts, it has been conducted for minimization of variation in A/F ratio to add an oxygen-storing substance (e.g. ceria or zirconia) to a noble metal (e.g. platinum, rhodium or palladium) having a catalytic activity, to allow the oxygen-storing substance to adsorb or desorb the oxygen present in exhaust gas.
These noble metals and oxygen-storing substances are present in a dispersed state in the pores of the porous .gamma.-alumina layer formed on the surfaces of the cell partition walls (ribs) of honeycomb structure.
As a specific example of the above improvements, there can be mentioned a honeycomb structure disclosed in JP-A-56-147637, wherein each cell has a triangular, rectangular or hexagonal sectional shape and each cell has a fillet at each angular portion (corner) . Also in JP-A-62-225250 is disclosed a honeycomb structure having hexagonal cells having a curve or R (radius: 1 mm or more) at each corner. Further in JP-A-7-39760 is disclosed a hexagonal-cell honeycomb structure wherein the upper and lower limits of bulk density are specified by setting the rib thickness at 0.05 to 0.150 mm and the porosity at 0.65 to 0.95; and in JP-A-8-193512 is disclosed a hexagonal-cell honeycomb structure (rib thickness: 0.17 mm, cell density: 62 cells/cm.sup.2) disposed close to an engine.
However, the hexagonal-cell honeycomb structure disclosed in JP-A-56-147637 is intended to avoid the excessive formation of layer (made of .gamma.-alumina or the like) in each corner of each cell and further to achieve the effective contact of exhaust gas with the noble metal loaded on .gamma.-alumina layer. Also, the hexagonal-cell honeycomb structure disclosed in JP-A-62-225250 is intended to avoid the peeling of the layer excessively formed in each corner of each cell, which may taken place owing to impact or thermal change; however, there is no description, in the Examples, on the rib thickness and cell density of the hexagonal-cell honeycomb structure.
Meanwhile, in the invention of hexagonal-cell honeycomb structure disclosed in JP-A-7-39760, it is intended to reduce the pressure loss by increasing the porosity, and quickly elevate the temperature of catalyst at the start of engine by decreasing the heat capacity of carrier. Also, in JP-A-8-193512, it is described that by disposing a hexagonal-cell honeycomb structure (which is superior in thermal impact resistance to a triangular- or rectangular-cell honeycomb structure) at a position close to engine (where the exhaust gas temperature is high), the warm-up property of catalyst can be improved. In the literature, it is also described that a triangular- or rectangular-cell honeycomb structure is preferred as a carrier for catalyst to be disposed at a position distant from engine, because such a honeycomb structure, as compared with a hexagonal-cell honeycomb structure, has a large GSA (geometric surface area) at the same cell density and, after the completion of warm-up, has high purifiability for exhaust gas.
Thus, the past improvements of catalysts were made in view of the purifiability for exhaust gas or the durability of catalytic performance and paid no attention to the strength of honeycomb structure, etc. Conventional honeycomb structures have consisted mainly of three kinds, i.e. triangular-cell, rectangular-cell and hexagonal-cell types.
Of these, a rectangular-cell type, particularly a square-cell type is used in a highest proportion. It is mainly because the square-cell type, as compared with other types, is well balanced in purifiability, pressure loss and strength and the die used in extrusion molding of square-cell honeycomb structure is easy to produce. These three types of honeycomb structures are ranked as shown in Table 1, when compared under the conditions of same rib thickness and same cell density.
TABLE 1 ______________________________________ Cell Purifia- Pressure Mechanical Thermal shock Overall Shape bility loss Strength resistance rating ______________________________________ Triangular .largecircle. X .largecircle. .DELTA. X Rectangular .circleincircle. .largecircle. .largecircle. .largecircle. .largecircle. Hexagonal .circleincircle. .circleincircle. .DELTA. .circleincircle. .DELTA. ______________________________________ .circleincircle.: excellent .largecircle.: good X: bad
Hexagonal-cell honeycomb structures are about equal to square-cell honeycomb structures in purifiability and superior to the latter in pressure loss; however, the former have low stiffness and low strength. Therefore, hexagonal-cell honeycomb structures have found no practical application as a carrier for catalyst for purification of automobile exhaust gas and their practical applications have been limited to stationary apparatuses requiring no large strength, such as carrier for catalyst for deodorization.